1. Field of the Invention
The present invention relates to an image processing apparatus and to an electrophotographic apparatus, such as a printer or a copier, that uses such an image processing apparatus, and relates in particular to an information processing apparatus, for converting into CMYK image data RGB image data that are optimized, for the characteristic of a display device, and for performing halftone processing to generate image reproduction data, and to an electrophotographic apparatus that uses this image processing apparatus. Either this, or the present invention relates to an image processing apparatus, for converting, for a first color space, image data that are optimized for a predetermined characteristic, which is not device dependent, to obtain image data for the color space for a toner used for an electrophotographic apparatus and for performing halftone processing by generating image reproduction data for expressing image halftones, and to an electrophotographic apparatus that uses this image processing apparatus.
The present application is based on Japanese Patent Applications No. Hei. 11-365499 and 2000-266897, which are incorporated herein by reference.
2. Description of the Related Art
To generate an image using computer graphics, an image is designed on the screen of a computer and RGB image data are generated. The RGB image data are tone data for pixels for individual colors, and the image is printed by an electrophotographic apparatus, such as a printer. The electrophotographic apparatus performs color conversion for the RGB image data to obtain CMYK image data that match a print engine and halftone processing to convert the thus obtained CMYK image data into image reproduction data for individual pixels. The image reproduction data are then supplied to the print engine, which uses a laser beam for printing.
A color electrophotographic apparatus employs CMYK toner, obtained by adding K (black) to CMY, but K (black) can also be provided by mixing together the CMY colors. Therefore, or a specific electrophotographic apparatus, the general rule, even in a CMY color space, is for image data to be generated for a CMYK color space, and the thus obtained image data used for printing. Therefore, the more generally applicatory CMYK color spaces are employed in the explanation given for this specification, and thus, again for this specification, the CMYK color space concept includes that for a CMY color space, which is substantially the same.
For a page printer that uses a laser beam, the image reproduction data are drive pulse width data that specify for a pixel a beam irradiation area, and a drive pulse for driving a laser beam is generated in accordance with the image reproduction data.
In the above color conversion, three-dimensional RGB data are converted into four-dimensional CMYK data (or three-dimensional CMY data). Normally, this color conversion process is performed by using a color conversion table that discretely represents the relationship between the RGB tone data and the CMYK tone data. But when included in the tone data are data for 256 tones for each of the RGB colors, an enormous amount of data (64 Mbytes) is required for combinations of CMYK tone data relative to combinations of RGB tone data, 2563=16700000 colors, and it is not realistic for all the CMYK tone data combinations to be listed in a color conversion table. Therefore, normally, for several hundreds to several tens of hundreds of colors selected from 2563=16700000 colors, the conversion values for grid points are included in a color conversion table, and intervals between the grid points are interpolated to obtain CMY conversion values. For this process, linear interpolation is generally employed.
A multi-level dithering method, which is a binary method for tone reproduction of a density modulated image, is widely employed for the halftone process. According to the multi-level dithering method, a halftone table (gamma table), which represents the correlation of tone data and image reproduction data, is examined to extract the tone data for individual CMYK colors, which are input signals, and a dot area, an area in a pixel to which toner is attached, is determined. This dot area is an area into which a laser beam for the attachment of toner is projected, and is designated by the image reproduction data (drive pulse width data). For a cell consisting of a plurality of adjacent pixels, these dots are used to form halftone spots in the cell, and the halftone of the density modulated image is reproduced in accordance with the sizes of the halftone spots.
The halftone table (gamma table), which is a conventional halftone conversion table, is used to convert the tone data for the individual CMYK colors to image reproduction data (drive pulse width data) that correspond to the optical densities obtained through printing. This conversion characteristic is generally a linear characteristic wherein, relative to the CMYK tone data, the drive pulse width is simply increased.
However, the output density of the monitor screen of a computer, e.g., a CRT screen, has a specific gamma characteristic relative to the RGB tone data. For example, FIG. 9 is a graph showing a characteristic that represents the relationship between RGB tone data x, for the CRT screen, and an output luminance I, on the display screen. According to this example, the ratio of the change in the luminance I to the change in the tone x is small in an area wherein the tone value corresponding to a dark image is low, and is large in an area wherein the tone value corresponding to a bright image is high. That is, I=kxn (n is normally 1.8 to 2.2).
The electrophotographic apparatus, such as a printer, outputs printed paper material. The luminance I, when the printed material is observed, is proportional to reflectivity R (I=k′R) of the printed material, and thus, R=kxn. The output density (optical density) D of the printed material is defined relative to the reflectivity R to establish the relationship D=−log10R.
FIG. 10 is a graph showing the characteristic that represents, according to the above equation, the relationship between the RGB tone value x on the monitor screen and the output density D of the printed material. According to this graph, the ratio of the change in the output density D to the change in the tone value x is large in an area wherein an input tone value x is low, and is small in an area wherein an input tone value x is high.
Therefore, since RGB tone data having the characteristic curve in FIG. 10 and linear conversion characteristic in the halftone processing are employed, the following problems have arisen. That is, in the color conversion, when the RGB tone data between the grid points are linearly interpolated to obtain CMYK tone data, and when the CMYK tone data are converted into image reproduction data (drive pulse width data) using the halftone process, the obtained image production data correspond to an output density that differs from an output density D (target reproduction density) for the RGB tone data shown in FIG. 10. This means that the output density of the image printed by the electrophotographic apparatus differs from the density that corresponds to the luminance of an image that is designed on the CRT screen of a computer. This is not preferable because the image on the CRT screen differs from the printed image and the image quality is deteriorated.
The above problem also arises with an electrophotographic apparatus that regards as an image RGB image data that are defined on a liquid crystal display and that employ CMYK ink. Similarly, a like problem has arisen with an electrophotographic apparatus that accepts as an input image sRGB image data defined by Windows®, CMYK image data generated by a printer, or CIELab or CIEXYZ image data constituting a color space (Device Independent Color Space) that, by using a color management module, is not device dependent, and that converts image data for the color space for a different ink.